Associate Professor of Organismal Biology Kennesaw State University, United States
Abstract: Fire-dependent longleaf pine (LLP) ecosystems are known for their open structure and hyperdiversity. In the last 200 years, these systems experienced widespread ecological state shift in part due to suppression of a frequent fire regime. In many areas fire suppression allows mesophyte species to encroach, creating less flammable environments with less biodiversity. Although prescribed fire is a commonly used restoration tool to reverse mesophication, fire behavior in mixed pine-mesophyte stands is poorly characterized. In this study we aimed to identify mechanisms by which canopy composition, structure, and fuels contribute to fire behavior, focusing on the understudied mountain LLP ecoregion in northwest Georgia. We hypothesized that woody vegetation composition and structure indirectly influence fire behavior through fuel layer traits. We predict relative decreases in basal area and increases in the relative importance of pine and pyrophytic hardwoods should promote fuel traits that increase fire rate of spread (RoS), and fuel consumption (FC). To test this, we collected fuel and fire data across stands varying in vegetation composition and structure during dormant season prescribed burns in 2022 and used Bayesian path analysis to estimate the effects of vegetation, fuel traits, and weather on fire behavior. We found that lower basal area and greater pine importance increased fine fuel load (β = .277 ± .208) and decreased fine fuel bulk density (β = -.232 ± .218). Fine fuel load decreased as mesophytic hardwood importance increased relative to pyrophytic hardwoods (β = -.377 ± .217), but bulk density was not affected (β = -.065 ± .243). As fuel load increased and bulk density decreased, RoS and FC increased (βRoS and FC~fuel load = .458 ± .252, βRoS and FC~bulk density =-.396 ± .255). Greater air temperature and lower relative humidity also increased RoS and FC to a similar degree as fuel traits (β = .370 ± .234). Indirect effects of woody vegetation on fire behavior were consistently weaker than direct effects, suggesting the possibility of canopy-derived traits other than fuels that may better explain fire behavior, or that other unexamined fuel traits are more important. Fire behavior in mountain LLP restoration is influenced by canopy composition and structure, factors that are easily estimated by land managers. While some mechanisms are unresolved, mesophytic hardwoods reduce fire intensity, in part by lowering available fuel load, and should be targeted for removal in restoration efforts.
